In many applications it is desirable to use exclusionary bearing or shaft seals to isolate a bearing from the environment or to isolate a closed area from the general environment. One well-known type of exclusion seal is the ferrofluid seal.
A conventional ferrofluid seal is comprised of a precisely dimensioned ring-shaped permanent magnet, ring-shaped pole pieces, and ferrofluid. The magnet is positioned around the shaft or bearing and the pole pieces are mechanically attached to the magnet faces and extend close to, but not touching, the shaft or bearing to form one or more gaps. The gaps are filled with ferrofluid.
The pole pieces are conventionally formed from a magnetically permeable metal and the ferrofluid is comprised of a suspension of magnetically permeable particles in a fluid carrier so that the pole pieces, the magnet, the ferrofluid and the shaft or bearing form a closed-loop magnetic circuit. The magnetic flux generated by the magnet passes through the ferrofluid and holds it in the gaps between the pole pieces and the shaft to form the seal. The construction and operation of such seals are described in detail in U.S. Pat. Nos. 4,407,508; 4,694,213; 4,630,943; 3,848,879; 4,628,384; 4,357,022 and 4,357,021 and will not be discussed further herein.
The current technique of manufacturing such ferrofluid seals generally involves several processing steps. First, the ring-shaped magnet is inserted into an assembly tool which holds the magnet. A bead of fast-curing adhesive (such as a cyanoacrylic adhesive) is then spread on one of the magnet faces. Finally, a metal pole piece is pressed against the magnet face into the adhesive to attach the pole piece to the magnet. In seal configurations in which pole pieces are attached to both magnet faces, the completed magnet and pole piece assembly is inverted and the above process is repeated.
As is obvious from the above-mentioned assembly steps, in the conventional construction, the metal pole piece is not an integral part of the magnet. Consequently, the integrity of the interface between the magnet and the pole piece depends on the integrity of the bond between the adhesive and the magnet and pole pieces. Due to bubbles and voids, the adhesive may have gaps which allow leaks in the magnet-pole piece interface and which, in turn, allow the entire seal to leak.
The conventional assembly technique is slow and expensive since several manufacturing steps are employed and time is required for the adhesive to cure at each step. Further it is difficult to manufacture seals with pole pieces that have non-planar geometries without machining the completed assembly. This machining is expensive and requires additional assembly time and steps. In addition, it is difficult to assemble seals which have very thin (less than 0.005 inch) pole pieces because dimensional tolerances cannot be maintained due to the variable thickness of the adhesive layer and due to distortion caused by handling.
Accordingly, it is an object of the present invention to provide a ferrofluid seal which can be constructed and inspected simply and with less steps than conventional ferrofluid seals thereby reducing cost.
It is another object of the present invention to provide a ferrofluid seal which is less prone to leakage than conventional ferrofluid seals.
It is yet a further object of the invention to provide a ferrofluid seal in which the pole pieces are formed of a material which directly bonds to the magnet without the use of any additional adhesive, thereby preventing leakage along the adhesive interface.
It is still another object of the present invention to provide a ferrofluid seal in which conventional metallic pole pieces are eliminated.
It is a further object of the present invention to provide a ferrofluid seal in which the pole pieces are formed of a ferromagnetic epoxy material which directly bonds to the magnet face.
It is another object of the invention to provide a ferrofluid seal in which intricate pole piece geometries can be constructed without substantial increase in the cost of the seal.
It is yet another object of the invention to provide a ferrofluid seal in which the magnetic field gradients in the seal can be conveniently controlled (through the shape of pole pieces thereby minimizing ferrofluid separation and changes in seal pressure capacity with time.
It is still another object of the invention to provide a ferrofluid seal in which the overall dimensional and tolerance control is superior due to trimming of the completed assembly.
It is yet a further object of the present invention to provide a ferrofluid seal in which any pole piece width may be achieved (such as very thin pole pieces) without distorting the part. Thus, seals with a very thin overall width can be produced.
It is yet another object of the present invention to provide a ferrofluid seal with epoxy pole pieces in which the pole pieces are electrically conducting. Thus the seals can be used with an electrically conducting ferrofluid to conduct electrical charges from a rotating member to a fixed member.